How to calculate gas top up (R22)

[DannySwain]

I have bought a Fujitsu Air Conditioner (AOY17ANA outdoor unit and ASY17ASH indoor wall unit) which I have successfully install in our bedroom. The unit was pumped down and removed some years age by a professional engineer.
I have vaced it down and did a test run and it seams to work OK! but I don't know how much gas is in the unit. I have calculated that it needs 1390grams of R22, but how do I calculate how much extra to put in, is there an easy way to do this ie; from the temperature and pressure at the charging port? or is it more complicated that this? (I am sure it is!)
I am a heating engineer by trade, and want to venture into aircon soon but I want to get this sorded asap.
I was told the best way is to dump the gas and start from scratch, but I think that this method leaves much to be desired and I don't have a recovery unit. Any Help would be useful
Regards Danny

[benijoseph]

Mr Danny,

When already unit is filled with Gas dont worry about

weight of the gas , check max AMPs unit can take ,then you

connect 2 guage one on suction side and one on di

scharge side , At 36c ambient suction guage must show

72 psi and discharge must be around 290 psi , it might

vary depending upon ambient and efficiency of the comp

ressor. hope this will solve your problem.

BENI JOSEPH.

[chemi-cool]

At 36c ambient

Hi Beni,

You made me laugh
Danny lives in England, he might have to wait a few years to get such high ambient.

It would be easier to charge untill the suction line entering the compressor will start to sweat.

In minisplits you can have a bit less or more refrigerant with no problems.

Chemi

[DannySwain]

I am a bit confused how do I "connect 2 gauges one on suction side and one on discharge side" When I only have the one Charging port! I am sure that ther must be a method that engineers use without too much grief!
Thanks for your replies
Regards Danny

[Morph]

I am a bit confused how do I "connect 2 gauges one on suction side and one on discharge side" When I only have the one Charging port! I am sure that ther must be a method that engineers use without too much grief!
Thanks for your replies
Regards Danny

Hi Danny I think what you are looking for are called 'line tap access valves'.

Three types I know of are 'Watsco', 'Supco' and 'A1' all do basically the same job, like it says on the box 'line access valve'.

Hope this helps
Morph

[Peter_1]

How, how ...stop guys.

Danny, how have you 'calculated' the charge? I'm eager to learn this. If you wants to venture in Refr or AC, then you first have to learn the basics, especially the log P diagram. Cooling apparently seems on the first sight almost he same as heating but it isn't at all. It's much more complicated then heating.
Dumping is by law since years no longer allowed, especially R22.

Beni, measuring the AMPS for what reason? Your theory is only valid if temperatures are correct, if unit was designed according the specs you give here, if there is no moisture in the system, if there is no air trapped in the system, .....

We use the same method as Chemi and crosscheck superheat and subcool.

[Temprite]

It would be easier to charge untill the suction line entering the compressor will start to sweat.
Chemi

Beer can cold.

[frank]

'line tap access valves'

Hi Morph

You would only need to apply a line tap valve when there is no way to connect your gauges, i.e. on a sealed system such as a domestic fridge.

These things are prone to leak and it would be foolish to put one on a split system when there are other means of attaching gauges.

To measure discharge pressure on a split you should remove the top from the condensing unit and 99 times out of 100 you will find a schaeder valve at the top of the condenser.

[Morph]

Hi Morph

You would only need to apply a line tap valve when there is no way to connect your gauges, i.e. on a sealed system such as a domestic fridge.

These things are prone to leak and it would be foolish to put one on a split system when there are other means of attaching gauges.

To measure discharge pressure on a split you should remove the top from the condensing unit and 99 times out of 100 you will find a schaeder valve at the top of the condenser.

oooOOOPPPPss well thats what you get for reading too many books at college and not getting enough pratical.

Thanks for that Frank
Morph

[bobjob]

Hi Danny

How did you calculate that your system needs 1390grams?

From an old Controls Centers catalogue the information given shows that the Fujitsu AOY17A/ ASY 17ASY wall mounted system has a R22 charge of 1080 grams.
This is for 5metres pipe length and the extra charge is 15grams per metre for a maximum pipe length of 20metres.

Refrigerant charge at max length would then be 1080+225 = 1305 grams


You really need to recover the R22 and weigh the right amount into the system.
This is the only way of doing it correctly.

This Fujitsu cooling unit like many other makes has been designed for an outdoor temperature of 35C DB. with an indoor temp of 27c DB

So with the ambient temperature in the UK you will sometimes see that the suction pipe is close to freezing over ( unless the unit is fitted with head pressure control )

Generally you can tell if the unit is undercharged :
a)Exspansion line iceing over
b)Poor T/D across the whole of the evap coil.

Regards Bob

[Brian_UK]

Danny, I would agree with Bob regards your calculation, but the easiest way would be to check the data plate on the condensing unit. This should show the base charge for the system.

Also, as previously said, DO NOT DUMP any gas.

If you have vacced and tested we assume that you have a set of test gauges, in which case, what are your readings and what are the air off temperatures etc.

[DannySwain]

I calculated the charge from the Manual that i downloaded from the Fujitsu website which stated 1350 grams for 5 metres of pipe plus 10 grams for each attional metre of pipe length. I have 9 metres So I arrived at 1390 grams. The engineer told me that as the unit had been standing around for some years it would have probably lost some of its charge and would need to be topped up!
What should the TD be across the condenser coils?
What is sweating exactly (does that mean droplets of water on the pipe)?
Whats a good book for beginers?
Thanks again for all your replies
Regards Danny

[Morph]

What should the TD be across the condenser coils?
What is sweating exactly (does that mean droplets of water on the pipe)?
Whats a good book for beginers?
Regards Danny

Hi Danny, I think I can answer this one, but if I'm wrong Frank will tell me. HI Frank

1)TD across condensor coils air-on minus air-off(5 to 10) C
condensing temp as seen on gauges/comparator is generally 15C above ambient.

2)Sweating of suction pipe you should be able to see condensation starting at compressor and stretching back to evaporator.

3)I'm a relative newby to fridge myself but from the books I have read I find the ones by Kotza to be very good. http://www.kotza.com/

Think I paid around £70 for both books from http://www.srwonline.co.uk

Hope this helps
Morph

[frank]

2)Sweating of suction pipe you should be able to see condensation starting at compressor and stretching back to evaporator.

Hi Morph

I would hope that the suction line was insulated, so you sould only see the sweating around the uninsulated suction pipe connection to the compressor. This only happens when you have sufficient load on the evaporator, the gas charge is correct and the system is in balance (heat energy rejected = heat energy absorbed + heat of compression)

[DannySwain]

I do thank you for all your replies, but I am still no satisfied with the responses! What is the algorythm (method) to calculate the required gas! I am sure that you guys have the formular! (temp-pressure etc (maybe) I have had so many conflicting information (not from you!) and am no more informed than I was last week!
As a heating engineer we have a good publication which gives very good information on all aspects of gas valves etc etc?(http://www.vipergas.co.uk/) Is there a guide like this for aircon?
Thanks for all your input
Regards Danny

[Brian_UK]

Danny, unfortunately natural or LP gas valves are not quite like refrigerant as regards measuring gas flow rates.

Most manufacturers will provide the data relating to the quanities of refrigerant required for their items of plant, very few (if any) techs have to calculate how much gas to put into a system starting from scratch. As you have found out already by measuring the liquid line pipe length and adding this to the amount for the condensing unit you have arrived at an answer.

If the manufacturer has said that is what is required then believe it.

You could check the correct operation by measuring the running pressures and system temperatures to ensure that they are within accepted parameters and if not then you may need to perform extra checks.

Readings required would cover:-
Suction pressure/temperature
Suction superheat
Discharge pressure/temperature
Condenser air on/air off
Liquid subcooling
Evaporator air on/air off db/wb
and so on.

It's a bit like the old saying.... If it ain't broke, don't fix it !

[DannySwain]

Well it looks like I need more expierence! (learn to walk before I can run) But I will keep at it and one day I may be able to help somebody on this forum myself
Thanks again to all of you for your replys
Regards Danny

[Brian_UK]

Don't dispair Danny the fact that you are asking questions means you're half way there...

Some additional reading if you want :-

http://www.fieldpiece.com/tech-articles-superheat.htm

also, this link www.jbind.com/Catalog/superheat/sh_hookup.html
has some piccys showing gauge hook-ups, although remember that it is advertising their products so instead of the expensive electronic gauge substitute your 'cheaper' model

Cheers

[DannySwain]

Thanks Brian, that's the sort of thing I was looking for!
Regards Danny

[chemi-cool]

Even the longest trip, starts with a little step.

If you want to work if our field, you will also need to know WHY, and HOW.

back to school my friend, the best place to study.

Chemi

[garybratton]

charge it to about 55 psi suction pressure

[botrous]

charge it to about 55 psi suction pressure

Nice one !!! How things are easy and simple and we just try to complicate it and our lives

[DannySwain]

charge it to about 55 psi suction pressure

I have just come to the conclusion that if I only have the 1 service charging valve on the suction side and for example the static pressure (compressor not running is say 100 psi @14C on the suction side), and when the compressor is running I have say 55psi! on the suction side., Then it would follow that the pressure on the Liquid side must be 100psi minus 55psi = 45psi plus 100psi equaling 145psi !!!! High Side!. This coupled with temp information I should be able to calculate the volume of gas in the unit????????
Or am I talking nonsense (think about it!)
Regards Danny

[chemi-cool]

Or am I talking nonsense (think about it!)

I will go for it


Chemi

[frank]

charge it to about 55 psi suction pressure

Do you know what temperature that equates to?

Would that be gauge pressure or absolute?

[botrous]

I have just come to the conclusion that if I only have the 1 service charging valve on the suction side and for example the static pressure (compressor not running is say 100 psi @14C on the suction side), and when the compressor is running I have say 55psi! on the suction side., Then it would follow that the pressure on the Liquid side must be 100psi minus 55psi = 45psi plus 100psi equaling 145psi !!!! High Side!. This coupled with temp information I should be able to calculate the volume of gas in the unit????????
Or am I talking nonsense (think about it!)
Regards Danny

Using the pressure tempreture chart (Log p - Tempreture chart) you can draw the cycle with the paramaters you mentioned .You have to know many other parameters to know the refrigerant mass , such as the ammout of energy you need to evacuate from the space to cool

[Brian_UK]

....Or am I talking nonsense ....

Who are we to say such a thing , however.....

[eggs]

danny, to change the subject slightly.
what you are asking is akin, to someone asking you. " is this old gas fire, i found in an old garage working right?"
I suspect you would say to us. " stop pissing about, and buy yourself a new gas fire."
i personally would not want, or have, a second hand R22 fujitsu wall mount in my bedroom.
1) it IS very noisy
2) it IS un-economical to run
3) it WILL pack in soon.
Do yourself a favour, spend £400 and fit yourself a new daikin inverter.

If you insist on keeping an old piece of history running, yes charge to 55-60 psi on the suction on a nice sunny day.

cheers

eggs

[DannySwain]

My Fujitsu is running fine thanks! (In fact I used it last night as it was hot and humid!) for the amount of use it will get in the UK it will be economical enough for my use and yes it is quiet.

I was talking theory, if you look back over the threads I was trying to find information on how to evaluate any unit as regards to topping up when you do not know the history of that unit and need information re pressures temp etc. I am begining to understand more and more about the refrigeration proccess and hope that others reading this may also gain a better understanding of the above etc.

I know I have a long way to go, but I will get there!
Regards Danny

[DannySwain]

Using the pressure tempreture chart (Log p - Tempreture chart) you can draw the cycle with the paramaters you mentioned .You have to know many other parameters to know the refrigerant mass , such as the ammout of energy you need to evacuate from the space to cool

I have yet to get my brain around that bit! But I am enjoying the learning curve!

[DannySwain]

Do you know what temperature that equates to?

Would that be gauge pressure or absolute?

No not as yet (I am working on it!) and I would think that it must be gauge pressure correct me if I am wrong!
Regards Danny

[frank]

Hi Danny

Spend a bit of money and buy a refrigerant comparator. That will show you the relationship between pressure and temperature for given refrigerants. When you charge a system you should be charging to temperatures not pressures.

[botrous]

I have yet to get my brain around that bit! But I am enjoying the learning curve!

Than you must be on the right way